1. Field of the Invention
This invention relates to a safety system for an operator of a machine tool and, in particular, to a safety system which utilizes a capacitively coupled transmission arrangement to provide a uniform pattern in a predetermined sensitivity zone around the machine tool.
2. Description of the Prior Art
Power driven machine tools constitute a primary cause of industrial accidents through crippling or severance of the hands or fingers of the machine tool operator. One attempted solution to this problem is the utilization of mechanical barriers to shield especially dangerous areas on the machine tool and prevent the introduction of any portion of the operator's anatomy into those areas of the tool while still allowing entry of the workpiece. For example, in the case of a pipe flanging tool, a barrier plate having an aperture sized to generally match the outside diameter of a tubular workpiece may be used to prevent the insertion of the operator's fingers or thumb into proximity with the jaws of the flanging machine. To be more fully certain that the operator's hands are protected, it is necessary that the aperture closely match the outside diameter of the workpiece. However, the aperture in the barrier plate must not be so restricted as to prevent the insertion of various sized or odd-shaped workpieces into the machine tool. But if the aperture is sized to receive a wide range of workpiece sizes, the possibility exists that a portion of the operator's body may become engaged by the jaws. It is difficult to find a barrier plate guard arrangement which is flexible enough to permit entry of a variety of workpiece sizes and shapes, yet which simultaneously affords protection to the operator and does not appreciably diminish the throughput of the machine tool.
Other mechanical expedients used in the art include the provision of barrier bars or touch bars which extend across the width of a machine tool. When struck by a portion of the operator's body these elements generate a signal which disables the machine tool. For example, in the case of the power rollers used to work rubber or elastomers, the operator stands on a platform facing two large counterrotating rollers which work the elastomeric material. Safety trip bars at knee level, elbow level and, perhaps even a head level trip wire, extend parallel to the axis of the rollers and interpose themselves between the body of the operator and the rollers. Thus if the operator becomes in any way snagged or drawn into the rotating machinery, the likelihood is that a portion of his body would engage against one of the trip members and would thereby disable the machine tool. However, it is possible that the restraints may be missed or, perhaps more likely, the restraints would terminate the operation of the machine tool only after injury has been inflicted upon the operator.
A number of radio controlled machine tool protection systems are available. Such systems include those described in the following U.S. Pat. Nos. 4,057,805 (Dowling, assigned to the assignee of the present invention); 3,983,483 (Pando); 3,950,755 (Westbrook, Sr.); 3,896,425 (Erichsen); 3,872,455 (Fuller et al.); 3,409,842 (Embling et al.); and 1,847,872 (Hand). Each of these devices utilizes some variant of an inductively coupled radio frequency transmitter-receiver arrangement. In such systems the sensitivity pattern of the receiving antenna is dependent upon the orientation of the transmitter on the person of the operator and is also subject to null spaces or voids in coverage. These systems detect and trip if material must be fed into the hazard zone surrounding the machine tool.
It is believed to be advantageous to provide a machine tool safety system which utilizes a capacitively coupled electric field generator (transmitter)-capacitive receptor antenna arrangement to eliminate the existence of null spaces and voids in the sensitivity coverage which exists with an inductively coupled transmitter-receiver arrangement. It is believed to be of further advantage to provide the electric field generator (transmitter) in a form able to be conveniently carried upon some portion of the person of the operator, e.g., the wrist. It is also believed advantageous to provide a safety system the sensitivity of which monotonically increases as the distance between the electric field generator (transmitter) and the capacitive receptor antenna mounted on the machine tool decreases.